1. Field of the Invention
This invention relates to a vacuum degassing method and its apparatus for removing bubbles in a molten substance at elevated temperature such as molten glass or molten metal, particularly to a vacuum degassing method and an improvement of its apparatus which is effective in a process for continuously feeding molten substance at elevated temperature.
2. Discussion of the Related Art
Conventionally, as such a vacuum degassing apparatus, there is one disclosed, for instance, in Japanese Examined Patent Publication No. 4205/1969.
As shown in FIG. 7, this apparatus is employed in a process wherein molten glass G as a molten substance at elevated temperature in a melting tank 106, is degassed and is continuously fed to a successive treatment furnace. As shown in FIG. 7, a vacuum housing 100 which is sucked in vacuum, accommodates a vacuum degassing vessel 101. This vacuum degassing vessel 101 is connected to an uprising pipe 102 wherein the molten glass G rises as a molten substance at elevated temperature before degassing, and is introduced into the vacuum degassing vessel 101. The vacuum degassing vessel 101 is connected to a downfalling pipe 103 wherein the molten glass G after degassing falls and is led out to a successive treatment furnace. Casings 104 and 105 are provided around the uprising pipe 102 and the downfalling pipe 103, respectively, for insulatively coating the uprising pipe 102 and the downfalling pipe 103, which are connected to the vacuum housing 100.
Furthermore, the uprising pipe 102 and the downfalling pipe 103 are made of noble metals such as platinum, since temperatures of these pipes are elevated up to 1200.degree. to 1500.degree. C. by the molten glass G.
In the vacuum degassing apparatus of this kind, a pressure inside the vacuum degassing vessel 101, is reduced to 1/20 to 1/3 atmospheric pressure. Therefore, it is necessary to set a difference of levels of the molten glass G in the melting tank 106 and the molten glass G in the vacuum degassing vessel 101, to be approximately 3.5 m. Accordingly, since lengths of the uprising pipe 102 and the downfalling pipe 103 are prolonged, the thermal expansion quantities of the uprising pipe 102 and the downfalling pipe 103, are enlarged. Accordingly, the structure of the vacuum degassing apparatus becomes unstable and is devoid of safety.
Furthermore, since, in the vacuum degassing apparatus of this kind, the molten glass G is led from the uprising pipe 102 to the downfalling pipe 103 only by decompression in the vacuum degassing vessel 101, the flow control of the molten glass G is difficult.